chemical thermodynamics of neptunium and plutonium - U.S. ...

584 A. Discussion of selected referencesIn agreement with other work, especially spectroscopic evidence, changes in activitycoefficient values alone can be used to explain these results.However, the chloride concentration had a marked effect on the reaction rate. Sinceno evidence has been obtained for complex formation between Np(V) and chloride, Cohen,Sullivan and Hindman [55COH/SUL] analysed the kinetic data only in terms ofNp(VI) chloride complex formation. Two different paths were examined: The formationof NpO 2 Cl + only, and the formation of both NpO 2 Cl + and NpO 2 Cl 2 (aq). Theexperiments were carried out at three temperatures: 0.0, 4.78 and 9.84 ◦ C.The data fit equally well up to a concentration of 1.5 M chloride by assuming eitherone or two complexes. Despite the large experimental uncertainties at high chlorideconcentrations, the data were best represented in this region by the mechanism involvingtwo chloride complexes. The authors, however, mention that the data couldprobably also be interpreted by an activity change as was observed with the correspondingnitrate system, rather than by the formation of a second chloride complex.For log 10 β 1 and log 10 β 2 , respectively, Cohen, Sullivan and Hindman [55COH/SUL]reported 0.10 and −0.80 at 0.0 ◦ C, 0.00 and −0.74 at 4.78 ◦ C, and −0.09 and −0.70at 9.84 ◦ C. We assume that the enthalpies of the two complexation reactions are constantwithin this temperature range and up to 25 ◦ C, and we obtain from these data r H m = −29.0 kJ·mol −1 for the formation reaction of NpO 2 Cl + ,and r H m =+15.3 kJ·mol −1 for the overall formation reaction of NpO 2 Cl 2 (aq). Using these enthalpiesto calculate the equilibrium constants at 25 ◦ C yields log 10 β 1 (298.15 K) =−0.37 and log 10 β 2 (298.15 K) =−0.56. We use only the first value in our evaluationand assign it an uncertainty of ±0.20. The second value is probably due to a mediumeffect. It should be noted that the IAEA review [92FUG/KHO] obtained approximatelythe same value for log 10 β 1 (298.15 K) but −0.94 for log 10 β 2 (298.15 K) probablydue to a transcription error (interchanging the log 10 β 2 values at 0.0 and 9.84 ◦ C).Also, it seems that the two enthalpy and the two entropy values given in the table of[92FUG/KHO, p.81] all carry the wrong sign.[55RAB/COW]Rabideau and co-workers [51RAB/LEM, 55RAB/COW] studied chloride complexationand disproportionation of plutonium(IV) in aqueous solutions of I =1MandI = 2 M, respectively, containing HCl/NaCl and HClO 4 . From the difference of theformal potentials in chloride and perchlorate media, the authors calculated the complexformation constant of PuCl 3+ , assuming that no plutonium(III) chloride complexeswere formed. However, the formation of PuCl 2+ may have a significant influenceon the redox equilibrium measured. The discrepancy between the formationconstant of PuCl 3+ of log 10 β 1 =−0.23 measured here [55RAB/COW] and, for example,the value of 0.15 obtained in [76BAG/RAM2] in the same medium (I =2M)may be explained by the formation of PuCl 2+ in the study of Rabideau and Cowan[55RAB/COW]. In this review, we correct the formation constant of PuCl 3+ obtainedat I =1 M and [Cl] t = 1 M for the formation of PuCl 2+ . For the formation constantof PuCl 2+ we use the value of Ward and Welch [56WAR/WEL] atI = 1 M (HCl):log 10 β 1 =−(0.04 ± 0.10), cf.Table18.3. The fact that this value may include the ef-